The beauty of the R600 design is that the chip is really modular and you can turn on or off entire parts of the chip. We already wrote that it will have less Shaders and possible even less ROPs than the real R600 chip but the R600 PRO will also feature a new slower memory interface.
This is supposed to be a more expensive mainstream card that should sell for $200 to $250.
The R600 PRO will have 256 bit memory and it will use GDDR4 at high clock speeds which should be enough to beat the Geforce 8600 GTS, even the overclocked ones.
The Radeon HD 2900 PRO will simply have more memory bandwidth, but the chip will be as big as R600 and will eat a lot of power. At the end it will bring you the performance, but it will be an expensive card for ATI to produce.